Mails include postal mails and express mails. Generally, external packings and inclusions of mails are paper; non-paper articles may be included in mails; and even prohibited goods may be transferred by mail. Existing methods for detecting prohibited goods in mails mainly include: (1) using a metal detector to detect if the mails contain metal articles; (2) using X-rays to image mails and judging if the mails contain suspicious articles; (3) detecting if the mails contain poisons such as cocaine, marihuana and heroin through the sensitive smell of drug detector dogs.
However, the first mode applies to prohibited metal goods only; by the second mode, it can be only determined if there are suspicious articles, but participation of human or judgment by further detection measures is needed to judge whether the suspicious articles are prohibited goods, and X-rays are greatly harmful to human bodies; and the third mode applies to poisons only, and can be executed only after opening the mail. Therefore, those mail detection modes are limited in applicable scope, and the detection results are undesirable.
At present, terahertz technology has been still developing. Terahertz (THz in short) is a unit of frequency, which is equal to 1,000,000,000,000 Hz, and generally represents the frequency of electromagnetic waves. Terahertz waves refer to electromagnetic waves with frequencies in a range of 0.1 THz-10 THz, and like radio waves, terahertz waves can transmit the majority of non-polar materials, such as paper, wood and plastics. Besides, many materials show unique terahertz spectrum absorption characteristics, therefore categories of materials in mails can be identified through the terahertz spectrum absorption characteristics of materials.